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Abstract

The inelastic emission spectrum of a single fluorescent microsphere (bead) exhibits resonances arising from whispering gallery modes. Two beads in close proximity form a coupled bisphere. Coherent coupling arises from each bead’s evanescent field and leads to resonance splitting. Here we collect emission spectra of two coupled beads, with nearly identical diameters, as spacing between beads is varied. Using these size-matched beads allows us to probe resonance splitting under strong coupling conditions.

Photographs of fluorescence emission from two beads matched in size. The top photograph shows the beads in contact, and the bottom photograph shows the beads close together but not in contact. To aid visualization in this article, a thin green ring has been overlaid upon the moving bead’s zero-order diffraction ring. The vertical arrow on the left (right) in each photograph identifies the left (right) edge of the beads’ first-order diffraction rings. The horizontal spacing between these edges is three pixels wider for the bottom photograph, compared to the top photograph. Thus, in the bottom photograph the beads are separated by 3 pixels x 0.114 μm/pixel = 0.34 μm.